In the art of proximity sensing, there is a need to determine a type of an object, e.g., a metallic object or a non-metallic object, in a proximity to the sensor. Conventional sensors use a combination of the inductive and capacitive sensing to determine a type of detected object. Those sensors include at least two sensing units, which conduct capacitive sensing and inductive sensing respectively. This dual structure complicates the sensing system.
For example, U.S. 20070159185 describes security scanners with capacitance and magnetic sensor arrays. Similarly, U.S. Pat. No. 6,801,044 describes a system including a composite measuring unit composed of two identical and symmetrically arranged oscillation circuits with measurement elements in the form of identical and symmetrically arranged inductive coils and capacitor chips.
U.S. 20130106769 describes a mixed sensing method using alternate inductive and capacitive modes. The inductive mode and capacitive mode are realized by exciting the sensor in different modes. However, that method requires switching circuits, which reduces the detecting speed.